I did this once and it caused huge debate at the time. Generally I don't do this, and it is a risk doing it, but the interesting point was on the legality of it and whether judges understood how things work.
The scenario is that a copyright holder has an image on their site, maybe a cartoon or a picture or some such. It is included in their web pages, and maybe they have other text and perhaps advertising which makes the copyright holder money. All well and good.
Then a rogue site links to the image using an img tag in the html, showing the image within their web site.
There seem to be two ideas why this may not be legal.
1. Secondary infringement. There have been cases of web sites that index and link to usenet postings of illegal copies of files. The web site does not copy the files, but was found to be guilty of secondary infringement because they facilitated the access to the illegal copies.
2. Unauthorised publication of the image on the rogue web site, even though the web site does not actually do any copying.
Firstly, for the technically challenged, the way it works is the rogue web site includes a link to the image on the copyright holders web site. When someone visits the rogue web site they get the web page, and their browser automatically gets all the linked in images including one from the copyright holder's web site, and presents the page as a whole on the screen. The rogue web site does not ever actually copy the image, but can expect that the end users browser will do its stuff as normal.
So point 1: When the browser gets the rogue web page and then gets the image it asks the copyright holder's server for a copy of the image. The server sends one. One can assume that as the server belongs to teh copyright holder then it sending a copy of the image when requested makes that an authorised copy of the image. You go to a musician and say "can I have a free copy of your latest CD" and he hands it to you - that makes it an authorised, legal, copy. That means there is no primary infringement happening, it is not an illegal copy, so facilitating that copy being made by linking to it from the rogue site cannot be secondary infringement.
Point 2 is harder. The meaning of publication is clear in the print world. I am not sure what it means in the web. Even so, assuming for a moment that somehow the rogue site is considered to be a publication and even though all it has is a link it is somehow considered to be publishing the image that is linked, you have the question of whether it is an authorised publication.
Now it is worth considering how the browser gets the image. It asks the copyright holder's server for a copy of the image. But that is not all it says. It says "I would like a copy of that image please to include in rogue web page". It sends details of the page in which it is to be included with the request as a "referrer". Many web servers are in fact configured such that they do little more than just log the referrer, but it is actually there in the request. The choice of the copyright holder to ignore that information when handling the request is, well, their choice. If they choose to hand out a legal authorised copy even when told "to be included in rogue web site" surely that can be taken as making the "publication" of the image in that context authorised?
Contentious I know. Copying other people's stuff is not on. Telling someone where they can see it (legally) is allowed. This is bang in the middle where you are telling them where to see it in a way that is all automated. Why does the automation make it wrong? :-)
What is really silly is why any cases like this ever get to court. The copyright holder has total control on the image being served in this scenario. They only lose control if someone makes an illegal copy which is much more clear cut. They can deploy many techniques, some of which are stupidly simple, to ensure the image is not included in a rogue web site, including having their web server check the referrer. They can even serve a totally separate image which could be offensive (questionable if that is legal) or just has text with a suitably rude message or maybe an adverts they make money from. They have control. The rogue web site takes a huge risk if they link to someone else's image without their agreement, so it does not happen that often.
With such simple technical solutions to this, why on earth do questionable legal cases need to ever be considered by the courts?
It is much better to have nice clear cut law on this than grey areas, IMHO.